With the recent advancements in integrated circuit (IC) technology, ICs are becoming increasingly compact. With the reduction in size of transistors on the ICs, the power supply voltage provided to the ICs is also being reduced. As a result, the IC standards have shifted focus from 5 Volts (V) and 3.3 V power supply to lower voltages, such as 2.5 V and 1.8 V. However, new system components, based on the new standards of lower power supply, should be backward compatible with the existing system components operating on 3.3 V power supply. For this, various schemes offer mixed voltage integrated circuits, for example, level shifters, which enable a circuit operating at a low voltage to communicate with another circuit operating at a high voltage.
Generally, the level shifters use thick oxide transistors, i.e., dual oxide layer gate transistors that can tolerate higher voltage. For example, a 3.3V level shifter is designed using transistors that can tolerate up to 3.3V potential difference. However, the usage of thick oxide transistors often involves operational problems when operated at lower voltages. For example, the thick oxide transistors for 3.3V are not able to provide high speed operation for devices operated at 2.5V or 1.8V, such as high speed processors. In addition, the thick oxide transistors also cover substantial chip area when operated at low voltages. Further, the thick oxide transistors are not configurable with ICs that have core operating at low power supply as the threshold voltage required to operate the thick oxide transistors is more than the available low power supply.
Thus, certain schemes have been devised to implement higher voltage level shifters using transistors recommended for low voltage application. For example, 32 Å thick oxide transistors have been implemented in the 3.3V level shifters. However, the 32 Å thick oxide transistors work at lower supply voltages, e.g., 1.8 V and may not be able to support high voltage overdrives, thus causing reliability issues. Some schemes suggest the use of a level shifter, which has transistors in cascode circuit arrangements to limit voltage stress on transistors operating as switching transistors. Such a cascode circuit arrangement of 32 Å thick oxide transistors generally operates either on the basis of an internally generated reference or at a low supply voltage as a biasing voltage to avoid stress. However, the level shifter may not be able to operate when the difference between an output high voltage and the biasing voltage is low. Further, the use of a reference voltage generator for providing an internally generated reference voltage results in increase in the chip area and may lead to leakage of current from the generator. Additionally, it is often observed that the problem of stress on the switching transistors increases with increase in switching frequency.